The present invention relates to a developer composition which is capable of providing visible images through reaction with an electron donating color precursor and which is useful in providing transparencies or in providing copies having a glossy or semi-glossy finish. More particularly, the present invention provides a developer composition which is resistant to blocking when stored in a vinyl binder.
The developer sheet of the present invention can be used in conjunction with imaging systems in which visible images are formed by image-wise transferring a color precursor to a developer sheet. These imaging systems include conventional pressure-sensitive copy paper, photosensitive imaging systems and thermal recording papers.
Pressure-sensitive copy paper is well known in the art. It is described in U S. Pat. Nos. 2,550,446; 2,712,507; 2,703,456; 3,016,308; 3,170,809; 3,455,721; 3,466,184; 3,672,935; 3,955,025; and 3,981,523.
Photosensitive imaging systems employing microencapsulated radiation sensitive compositions containing color precursors are described in commonly assigned U.S. Pat. Nos. 4,399,209 and 4,440,846, among others. In its most typical embodiments, the photosensitive composition is a photopolymerizable composition including a polyethylenically unsaturated compound and a photoinitiator which is encapsulated with a color precursor. Exposure of the microcapsules to actinic radiation hardens the internal phase. Following the exposure, the imaging sheet is subjected to pressure in contact with a developer sheet by passage through the nip between a pair of pressure rollers whereupon the color precursor is image-wise transferred to the developer sheet where it reacts to form the image.
Thermal transfer systems are described in Japanese Published Application 62-60694.
In applications in which the aforementioned imaging systems are used to reproduce photographic quality images, a high degree of gloss is often desired in the reproduction. Where a transparency is desired, the reproduction must also transmit light efficiently. These objectives are difficult to achieve using conventional developers.
Developers which are capable of glossing are described in commonly assigned U.S. application Ser. No. 905,727, filed Sept. 9, 1986, now abandoned, and U.S. application Ser. No. 086,059, filed Aug. 14, 1987, now U.S. Pat. No. 4,877,767. Both applications describe a developer sheet having a layer of a finely divided thermoplastic acidic resin developer on the surface. The capillary network formed by the interstitial spaces among the finely divided resin particles functions to draw the color precursor into the developer layer where the resin reacts with the color precursor to form the image. Subsequent heating coalesces the resin into a glossimparting film. In the former application the resins are phenolic resins. In the latter application they are acrylic microparticles formed by emulsion or suspension polymerization with a core-shell construction which provides unique melting characteristics.
One of the drawbacks of previous glossable developer materials is a tendency to block when they are stored in a vinyl binder. This blocking is due to plasticizers in the vinyl, such as dibutyl phthalate, dioctyl phthalate, and tricresyl phthalate, migrating from the vinyl binder cover to the developer layer and softening the developer composition. Over a period of several weeks, sufficient vinyl plasticizer has diffused from the binder to weaken the developer layer to such an extent that the adhesive strength between the developer layer and the binder becomes greater than the cohesive strength of the developer layer. When this occurs pick-off is observed as the developer sheet is separated from the vinyl.
An improved developer composition is described in commonly assigned U.S. application Ser. No. 152,685, filed Feb. 5, 1988, now U.S. Pat. No. 4,853,364, wherein the resins described in the aforementioned applications are combined. The phenolic resins described in the former application provide good density but have a tendency to be tacky and exhibit a high yellowing index, poor adhesion and cohesion, and tend to scratch easily. The acrylic developers described in the latter application exhibit good abrasion resistance but generally do not provide as high density as can be achieved with the phenolic resins. By modifying the acrylic resin to make it compatible with the phenolic resin and combining the two resins, a developer composition is achieved having the combined properties of good density and adhesive and cohesive strength. Even this composition, however, does not provide the vinyl blocking resistance which would be desired.